A power compensator is used to control reactive power of a high voltage electrical power system. The power compensator is able to produce as well as absorb reactive power. The power compensator comprises a voltage source converter (VSC) with a dc side and an ac side. The ac side of the voltage source converter is connected to the high voltage electrical power system. To be capable to control the reactive power, the dc side of the voltage source converter is provided with a dc power source.
Today the dc power source is a high voltage battery. Since the power apparatus is connected to the ac voltage of the electrical high power system, a plurality of battery cells have to be connected in series to match the dc voltage of the power apparatus. Moreover, to obtain the desired active power and duration of the energy storage, a number of strings including a plurality of battery cells must be connected in parallel.
Furthermore, in case of a fault, such as a short-circuit, the power apparatus has to be protected. Consequently, two switches are provided in each string to disconnect the string in case the converter, a string, or a part of the string is short-circuited. One switch is able to disconnect the string from the positive dc rail of the converter and the other switch is able to disconnect the string from the negative dc rail of the converter. Each of the switches has to be rated for the entire converter voltage to be capable of protecting the power apparatus.
Up to now, mechanical dc circuit breakers have been used as switches for connection and disconnection of the strings. However, the availability of dc circuit breakers is limited and the ratings of these breakers are considerably low and less than the required voltage ratings in high voltage electrical power systems. To overcome this problem mechanical ac circuit breakers with some additional circuitry, such as a resonance circuit connected in parallel with the ac circuit breaker, have been used. The additional circuitry makes the mechanical ac circuit breakers less robust, expensive and space requiring as well as being extremely sensitive to circuit parameters.
A solid-state switch is an electronic switch that, unlike a mechanical breaker, contains no moving parts. There is a desire on the market to be able to exchange the mechanical ac circuit breakers with solid-state switches. However, solid-state switches with a voltage rating high enough to replace the mechanical ac circuit breakers of the power apparatus for connecting and disconnecting the strings of a power apparatus for a high voltage electrical power system are not available on the market.